1. Field of the Invention
The present invention relates generally to a transfer apparatus which is capable of transferring a lead frame having encapsulated chips to a cut/forming stage during the manufacture of semiconductor packages. More particularly, the present invention relates to a transfer apparatus for transferring lead frames, which is provided with sliding blocks, the position of which can be easily adjusted depending on the lead frame to be unloaded.
2. Description of the Related Arts
In a common assembly process for semiconductor chip packages, an individual chip is separated from a wafer and then attached to a die pad of a lead frame using an adhesive, e.g., Ag-epoxy adhesive ("Die Attach"). The die pad is then electrically connected to inner leads of a lead frame ("Lead Bonding"). Further, the chip, together with the electrical connections, is encapsulated using a molding compound to provide protection from external environmental stresses such as moisture, dust, chemical and physical impacts.
During the assembly process, a number of the semi-assembled packages arranged on a lead frame strip are stored and carried as a stack in a carrier such as a magazine. The lead frame strip (also referred to as "lead frame") having a plurality of semi-assembled packages (also referred to as "molding parts") is subjected to a trimming and forming process wherein leads extending from the molding part are trimmed and formed to have a suitable shape, for example a J-shape or a gull-wing. In order to transfer the lead frame from the magazine to the trimming/forming stage, a lead frame transfer apparatus is employed.
The lead frame transfer apparatus comprises at least one tool for picking up the lead frame (a "picker") by vacuum pressure and transferring it to the trim/form stage. At the trim/form stage, the semiconductor chip and lead frames are trimmed from the rest of the lead frame strip and leads are formed into their final configuration for PWB assembly. Then, marking and branding steps are carried out.
Usually, the packages have different shapes and thus the lead frames of the same widths may have different forming positions. Therefore, appropriate modifications must be made to the existing lead frame transfer apparatus, or a new lead frame transfer apparatus must be employed.
FIG. 1 is a perspective view of a conventional lead frame transfer apparatus; FIG. 2 is a perspective view of an unloading tool of the conventional lead frame transfer apparatus of FIG. 1; and FIG. 3 shows a picker assembly for the lead frame transfer apparatus of FIG. 2.
Referring to FIGS. 1 through 3, the lead frame transfer apparatus 400 comprises a magazine where a plurality of lead frames 5 are stacked and stored. An elevator 200 receives the magazine and lifts the individual lead frame 5 stacked in the magazine to a predetermined height. An unloading tool 100 is provided with pickers for picking up the elevated lead frame 5. A transfer bar 300 is mechanically fixed to the unloading tool 100 at one end and transfers the unloaded lead frame to a trim/form stage with the aid of a power driving means such as a cylinder fixed to the other end of the transfer bar 300.
The movement of all the parts of the apparatus 400 is accomplished by the action of the transfer bar 300, except the vacuum unloading of lead frames 5 by the unloading tool 100.
The unloading tool 100 shown in FIG. 2 has a body 30 provided with a partition wall 10 at its center, and a plurality of suction means (e.g., four pickers 90) fixed through the holes 22 to a bottom fixing side 20 of the body 30. While four pickers 90 are shown in FIG. 1, for simplicity and ease of reference only one picker 90 is shown in FIG. 2.
The body 30 is open at its top, and has a bottom fixing side 20 whose thickness is greater than those of the other sides. The picker 90 is firmly fixed to the fixing side 20.
With reference to FIG. 3, the picker 90 is shown in greater detail and generally consists of three parts: a screw part 40, vacuum suction means 60 and vacuum suction inlet part 70, and washer part 43. The picker 90 also has a nut 65, a spring 68 for supporting the screw, and a stopper 63.
The screw 40 has a groove 42 at its head, and a hollow projection 48 which is inserted into one end of a tube 41 connected to a vacuum means, for example a vacuum pump (not shown). The screw 40 also has a screw thread 46. The screw 40 is hollow from the top to the bottom, and is sometimes called a "vacuum screw".
The vacuum suction means 60 has a cylindrical shape, and is provided with internal spiral grooves 64 corresponding to the screw thread 46, as well as external spiral grooves 66 at its outside surface. The vacuum suction means 60 also has, at its lower part under the spring 68, a groove into which the spring stopper 63 is engaged. The vacuum suction inlet 70 is integral with the lowermost part of the vacuum suction means 60.
In more detail, the picker 90 is fixed to the fixing side 20 of the unloading tool 100 shown in FIG. 2 by the steps of: inserting the top of the vacuum suction means and vacuum suction inlet part into the hole 22 of the fixing side 20, fitting the washer 43 to the outside of the upper body 62, and screwing the screw head 46 of the screw 40 into the internal spiral grooves 64 of the vacuum suction means 60.
Then, the nut 65 is screwed on the external grooves 66 of the vacuum suction means 60 so as to firmly fix the picker to the fixing side 20. A spring 68, for preventing the screw from loosening, is fitted to the suction means 60, and the spring stopper 63 engages the groove 61 to fix the spring 68.
FIGS. 4 through 8 are a sequential diagram showing the unloading of a lead frame 5 using the conventional lead frame transfer apparatus. In operation, the transfer bar 300 fixed to the unloading tool 100 moves down until the vacuum suction inlet 70 of the picker 90 in FIG. 2 contacts the molding part 5' of the lead frame 5 in the magazine 200 with the aid of a driving means such as a cylinder (not shown). Then, as shown in FIG. 5, the molding part 5' of the lead frame is attached to the vacuum suction inlet 70 by a vacuum applied through picker 90. As shown in FIG. 6, the picker 90 moves upward together with lead frame 5 attached thereto, by the action of the transfer bar 300. The lead frame 5 is then transferred by the transfer bar 300 to a trim/form stage for the trim/form process (not shown).
The next lead frame 5 stacked in the magazine is then lifted by the elevator 200. Thereafter, the steps in FIGS. 4 through 6 are repeated. For stable unloading of the lead frame from the magazine, the pickers 90 are positioned so as to be aligned to the molding parts 5' of the unloaded lead frames 5.
FIGS. 7 and 8 show the unloading process of a lead frame using another conventional lead frame transfer apparatus. With reference to FIGS. 7 and 8, the lead frame 15 has molding parts 16 comprising a chip at positions different from those of the lead frame 5 shown in FIGS. 4 through 6. Since the pickers cannot be correctly aligned to the molding parts 16 of the lead frame 15, the pickers can not adequately pick up the lead frame 15, and in the worst case, the lead frame drops as shown in FIG. 8, resulting in the failure of an unloading process for the next lead frame 15 from the magazine.
Thus, the conventional lead frame transfer apparatus 400 reliably unloads the lead frame from the magazine only when its pickers 90 are correctly aligned to the molding parts 5' of the lead frame 5 as shown in FIGS. 4-6. Therefore, in order to unload the lead frame 15 shown in FIGS. 7 and 8, a new or modified unloading apparatus must be employed, resulting in a loss of time and money.